Antireflection coatings

ABSTRACT

A transparency includes a first substrate having a first surface and a second surface and a second substrate having a third surface and a fourth surface. An optical coating is positioned on the fourth surface and has a refractive index real component n of greater than about 1.6 and an nk ratio greater than about 0.4, both as measured at 550 nm. The optical coating is configured to attenuate the transmission of the second substrate and not substantially affect the reflectivity of the fourth surface, as viewed from the first surface, such that the attenuation factor is less than about 50%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/788,939, filed on Oct. 20, 2017, entitled ANTIREFLECTION COATINGS,which claims the benefit of and priority to U.S. Provisional PatentApplication No. 62/410,913, filed on Oct. 21, 2016, entitledANTIREFLECTION COATINGS, the entire disclosures of which are herebyincorporated herein by reference in their entirety.

This application is related to a U.S. patent application identifiedunder Attorney Docket No. AUTO 02228C2 GEN010 P922C, filed on the samedate as the present application, and entitled ANTIREFLECTION COATINGS,which is a divisional of U.S. patent application Ser. No. 15/788,939,filed on Oct. 20, 2017, entitled ANTIREFLECTION COATINGS, which claimsthe benefit of and priority to U.S. Provisional Patent Application No.62/410,913, filed on Oct. 21, 2016, entitled ANTIREFLECTION COATINGS,the entire disclosures of which are hereby incorporated herein byreference in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to transparencies, and moreparticularly, to transparencies incorporating antireflection coatings.

SUMMARY OF THE DISCLOSURE

According to an aspect of the present disclosure, a transparencyincludes a first substrate having a first surface and a second surface.A second substrate includes a third surface and a fourth surface. Anoptical coating is positioned on the fourth surface and has a refractiveindex real component n of greater than about 1.6 and an nk ratio greaterthan about 0.4, both as measured at 550 nm. The optical coating isconfigured to attenuate the transmission of the second substrate and notsubstantially affect the reflectivity of the fourth surface as viewedfrom the first surface of the transparency such that the attenuationfactor is less than about 50%.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a front perspective view of a heads-up display systemincorporating an electro-optic-element, according to one example;

FIG. 2 is a front perspective view of a heads-up display systemincorporating an electro-optic-element, according to another example;

FIG. 3 is a cross-sectional view of the electro-optic assembly of FIG. 1across line III;

FIG. 4 is a cross-sectional view of a second substrate;

FIG. 5 illustrates the reflectance versus wavelength dependence of ametallic and a dielectric AR coating compared to raw glass;

FIG. 6A is a contour plot of reflectance for nk ratio vs. n;

FIG. 6B is a contour plot of reflectance for nk ratio vs. n;

FIG. 7 is a graph of the fourth surface antireflection spectra;

FIG. 8 illustrates reflectance of different metal-based antireflectioncoatings as a function of coating thickness;

FIG. 9 illustrates a graph of reflectance vs. wavelength for differentoptical coatings;

FIG. 10 is a graph of reflectance vs. wavelength for severalmetal/dielectric optical coatings;

FIG. 11 is a graph of the fourth surface antireflection spectra;

FIG. 12 is a graph of reflectance vs. wavelength for severalmetal/diamond-like carbon optical coatings;

FIG. 13 is a contour plot of transmittance for nk ratio vs. n; and

FIG. 14 is a cross-sectional view of an electro-optic element, accordingto one example.

DETAILED DESCRIPTION

The present illustrated embodiments reside primarily in combinations ofmethod steps and apparatus components related to an electro-opticassembly, more particularly, a heads-up display system having anelectro-optic assembly. Accordingly, the apparatus components and methodsteps have been represented, where appropriate, by conventional symbolsin the drawings, showing only those specific details that are pertinentto understanding the embodiments of the present disclosure so as not toobscure the disclosure with details that will be readily apparent tothose of ordinary skill in the art having the benefit of the descriptionherein. Further, like numerals in the description and drawings representlike elements.

For purposes of description herein, the terms “upper,” “lower,” “right,”“left,” “rear,” “front,” “vertical,” “horizontal,” and derivativesthereof, shall relate to the disclosure as oriented in FIG. 1. Unlessstated otherwise, the term “front” shall refer to the surface of theelement closer to an intended viewer of the electro-optic heads-updisplay assembly, and the term “rear” shall refer to the surface of theelement further from the intended viewer of the electro-optic heads-updisplay system. However, it is to be understood that the disclosure mayassume various alternative orientations, except where expresslyspecified to the contrary. It is also to be understood that the specificdevices and processes illustrated in the attached drawings, anddescribed in the following specification are simply exemplaryembodiments of the inventive concepts defined in the appended claims.Hence, specific dimensions and other physical characteristics relatingto the embodiments disclosed herein are not to be considered aslimiting, unless the claims expressly state otherwise.

The terms “including,” “comprises,” “comprising,” or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus. An element preceded by “comprises a . . . ” does not, withoutmore constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

In regards to FIGS. 1-3, reference numeral 10 generally designates anelectro-optic assembly. The electro-optic assembly 10 may be utilized ina heads-up display system 14 of a vehicle 18. The electro-optic assembly10 can have a first partially reflective, partially transmissive glasssubstrate 22 and a second partially reflective, partially transmissiveglass substrate 26. The first substrate 22 can have a first surface 22Aand a second surface 22B. The second substrate 26 can have a thirdsurface 26A and a fourth surface 26B. The first and second substrates22, 26 can be positioned in a parallel spaced-apart relationship and canhave a seal 30 substantially around a perimeter of the first and secondsubstrates 22, 26. The first substrate 22 and the second substrate 26define a cavity 34. An electro-optic medium 38 is in the cavity 34between the first and second substrates 22, 26. In at least one example,the electro-optic assembly 10 is configured to have a non-varyingreflectance and a varying transmittance. A “clear state” of theelectro-optic assembly 10 refers to the condition of maximumtransmittance. The activation of the electro-optic medium 38 may reducethe transmittance of the electro-optic assembly 10 to a “darkenedstate.” The “low end” transmittance refers to the minimum transmittanceattainable by the electro-optic assembly 10.

By way of explanation and not limitation, the electro-optic assembly 10can be included in the heads-up display (HUD) system 14 of the vehicle18. In such an example, the electro-optic element 10 may function as acombiner screen to reflect a primary image projected by a projector 46.The electro-optic assembly 10 can be controlled to vary the amount oflight transmission based on input from a control circuit. For example,in daylight conditions the electro-optic assembly 10 may be darkened toimprove or increase the contrast ratio and allow for improved visibilityof information projected on the electro-optic assembly 10 from theprojector 46. The contrast ratio may represent the ratio of a primaryreflected image from the projector 46 and the light transmitted throughthe electro-optic assembly 10 (e.g., in either the clear state or thedarkened state).

The heads-up display system 14 is capable of use in a variety ofapplications, such as automotive and aerospace applications, to presentinformation to a driver or pilot while allowing simultaneous forwardvision. In some examples the heads-up display system 14 may be providedvehicle rearward of a windscreen 54 and protruding from an instrumentpanel 58 (FIG. 1) while in other examples the electro-optic assembly 10may be positioned directly on the windscreen 54 (FIG. 2). Theelectro-optic assembly 10 may be any size, shape, bend radius, angle orposition. The electro-optic assembly 10 can be used to display manyvehicle-related functions or driver assistance systems such as alerts,warnings or vehicle diagnostics. In the depicted examples, the speed ofthe vehicle 18 is being displayed on the electro-optic assembly 10.

In regards to heads-up display systems 14, the image projected onto theelectro-optic assembly 10 should be bright enough to see in anycondition. This is particularly challenging when the lighting outsidethe vehicle 18 is bright. The contrast between the light from theprojector 46 and the lighting behind the electro-optic assembly 10 canbe low on a bright sunny day. While a brighter, more intense lightingsource (e.g., the projector 46) improves the contrast, increasing thedisplay brightness may not be the most economical solution and a displaythat is bright enough to provide reasonable contrast in very brightdaylight conditions will be too bright in other conditions. Althoughcontrols may be used to deal with variations in brightness, the specificbackground is ever changing in a moving vehicle, and depends in part onthe position of the driver's eyes. In accordance with one example, theelectro-optic assembly 10 can be configured to lower the transmissionand/or to increase the contrast ratio.

Depending on the application, there may be a need for a higher or lowertransmittance in the clear state, different reflectance values foroptimal contrast ratios, and/or broader dynamic range of thetransmittance levels. The initial reflectance and range of transmittanceproperties is further complicated by the capabilities of the projector46 employed with the heads-up display system 14 and the light outputcapabilities of the projector 46 along with the light transmittancelevels for the windscreen 54. The windscreen 54 will have a directimpact on the contrast ratio and visibility of the image from theheads-up display system 14. There are a number of factors which affectthe transmittance levels of the windscreen 54. The minimum lighttransmittance is based on the rules in the location in which the vehicle18 is sold but higher transmittance levels may be present based on howthe vehicle 18 is equipped and marketed. This range of factors createsthe need for solutions which can be adapted to different vehicle andenvironmental conditions.

Another aspect that should be considered when utilizing the heads-updisplay system 14 is that of secondary reflections which may result fromthe non-transflective surfaces, such as the first through fourthsurfaces 22A-26B of the first and second substrates 22, 26. For example,if a transflective coating is on the first surface 22A, then thesecondary reflectances may originate from the second through fourthsurfaces 22B-26B. Alternatively, if the transflective coating is on thesecond surface 22B, then the secondary reflectances may originate fromthe first, third or fourth surfaces 22A, 26A, 26B. Reflection off of thefirst through fourth surfaces 22A-26B may create a double image effectfrom secondary reflections that do not perfectly align with the primaryreflected image (e.g., due to geometries of the components of theelectro-optic assembly 10). The double image that may be formed fromsecondary reflections off the first through fourth surfaces 22A-26B maycause the primary image projected by the projector 46 and reflected bythe transflective coating of the electro-optic assembly 10 to appearblurry or unclear.

According to one example, the electro-optic assembly 10 can be assembledusing two approximately 1.6 mm glass substrates (e.g., the first andsecond substrates 22, 26) which are both bent with a spherical radius ofapproximately 1250 mm. Other thicknesses or radii for the first andsecond substrates 22, 26 may be substituted without departing from theteachings provided herein. In other examples the first and secondsubstrates 22, 26 may be bent to have a “free-form” shape. The desiredshape is one in which the resultant primary reflected image “appears” tobe forward of the electro-optic assembly 10 and forward of the vehicle18. The exact surface contour needed to attain this characteristic is afunction of the properties of the projector 46 and driver location, aswell as the electro-optic assembly 10 location relative to the other twolocations. Having the image projected forward of the vehicle 18 allowsthe driver to obtain the desired information without having to changetheir focal distance. In a traditional heads-up display located withinthe vehicle 18, the driver's eyes often have to refocus to the shorterviewing distance thus decreasing the time spent viewing the road.Furthermore, the driver's eyes will also then have to re-focus on theroad ahead, which further decreases the time spent viewing the road andforward conditions. The shape of the electro-optic assembly 10 shouldalso be selected so as to preserve the basic characteristics of theprojected image (i.e., straight lines remain straight, aspect ratios ofimages are preserved, etc.).

Referring now to FIG. 3, the first substrate 22 includes the firstsurface 22A and the second surface 22B. The second surface 22B can becoated with indium tin oxide with a sheet resistance of approximately 12ohms/sq. The first surface 22A can be concave and can be coated withchromium (Cr). The coated first substrate 22 may have a transmission ofapproximately 37.8% and reflectance of approximately 25.4%. The secondsubstrate 26 defines the third and fourth surfaces 26A, 26B. The thirdsurface 26A can be coated with indium tin oxide with a sheet resistanceof approximately 12 ohms/sq.

From the first surface 22A, the electro-optic assembly 10 can have aclear state reflectance of approximately 25% and a transmittance ofapproximately 24%. The electro-optic assembly 10 can have a low end, orstate, transmittance of approximately 10.5% and a reflectance from thefirst surface 22A of approximately 15%. Alternatively, in otherexamples, the high end, or state, transmittance of the electro-opticassembly 10 may be greater than 45% or even 60%. The characteristics ofthe electro-optic assembly 10 may also be altered so that the low endtransmittance is less than 7.5% or even less than 5% in the darkenedstate. In some examples, transmittance levels down to 2.5% or less maybe desirable. Increasing the high-end transmittance may be obtained bythe use of coatings and materials which have low absorption. Lowerlow-end transmittances may optionally be obtained through the inclusionof materials which have higher absorption. If a wide dynamic range isdesired, then low absorption materials may be used in combination withelectro-optic materials and cell spacings (e.g., the space between thefirst and second substrates 22, 26) which attain higher absorbance inthe activated state. Those skilled in the art will recognize that thereexists a multitude of combinations of coatings and electro-opticmaterials, cell spacings and coating conductivity levels which can beselected to attain particular device characteristics.

Still referring to FIG. 3, to provide electric current to the first andsecond substrates 22, 26 and electro-optic medium 38, electricalelements may be provided on opposing sides of the first and secondsubstrates 22, 26 (e.g., the second and third surfaces 22B, 26A) togenerate an electrical potential therebetween. In one example, a J-clipmay be electrically engaged with each electrical element, and elementwires extend from the J-clips to a primary printed circuit board. Othercontact designs are possible including the use of conductive ink orepoxy.

According to various examples, the electro-optic medium 38 may be anelectrochromic medium and/or liquid crystal (e.g., a twisted nematic)medium/material. In electrochromic examples, the electro-optic medium 38may include at least one solvent, at least one anodic material, and atleast one cathodic material. Typically, both of the anodic and cathodicmaterials are electroactive and at least one of them is electrochromic.It will be understood that regardless of its ordinary meaning, the term“electroactive” may mean a material that undergoes a modification in itsoxidation state upon exposure to a particular electrical potentialdifference. Additionally, it will be understood that the term“electrochromic” may mean, regardless of its ordinary meaning, amaterial that exhibits a change in its extinction coefficient at one ormore wavelengths upon exposure to a particular electrical potentialdifference. Electrochromic components, as described herein, includematerials whose color or opacity are affected by electric current, suchthat when an electrical current is applied to the material, the color oropacity change from a first phase to a second phase. The electrochromiccomponent may be a single-layer, single-phase component, multi-layercomponent, or multi-phase component, as described in U.S. Pat. No.5,928,572 entitled “ELECTROCHROMIC LAYER AND DEVICES COMPRISING SAME,”U.S. Pat. No. 5,998,617 entitled “ELECTROCHROMIC COMPOUNDS,” U.S. Pat.No. 6,020,987 entitled “ELECTROCHROMIC MEDIUM CAPABLE OF PRODUCING APRE-SELECTED COLOR,” U.S. Pat. No. 6,037,471 entitled “ELECTROCHROMICCOMPOUNDS,” U.S. Pat. No. 6,141,137 entitled “ELECTROCHROMIC MEDIA FORPRODUCING A PRE-SELECTED COLOR,” U.S. Pat. No. 6,241,916 entitled“ELECTROCHROMIC SYSTEM,” U.S. Pat. No. 6,193,912 entitled “NEARINFRARED-ABSORBING ELECTROCHROMIC COMPOUNDS AND DEVICES COMPRISINGSAME,” U.S. Pat. No. 6,249,369 entitled “COUPLED ELECTROCHROMICCOMPOUNDS WITH PHOTOSTABLE DICATION OXIDATION STATES,” and U.S. Pat. No.6,137,620 entitled “ELECTROCHROMIC MEDIA WITH CONCENTRATION ENHANCEDSTABILITY, PROCESS FOR THE PREPARATION THEREOF AND USE IN ELECTROCHROMICDEVICES,” and U.S. Pat. No. 6,519,072 entitled “ELECTROCHROMIC DEVICE”;and International Publication Nos. WO 98/42796 entitled “ELECTROCHROMICPOLYMERIC SOLID FILMS, MANUFACTURING ELECTROCHROMIC DEVICES USING SUCHSOLID FILMS, AND PROCESSES FOR MAKING SUCH SOLID FILMS AND DEVICES,” andWO 99/02621 entitled “ELECTROCHROME POLYMER SYSTEM,” which are hereinincorporated by reference in their entirety. The first and secondsubstrates 22, 26 are not limited to glass elements but may also be anyother element having partially reflective, partially transmissiveproperties.

In the non-limiting, depicted example of FIG. 3, each of the first andsecond substrates 22, 26 include a rounded edge 62 and a contact edge 66that is not rounded. It will be understood that the finished assembly 10may or may not have the depicted configuration. The non-rounded contactedge 66 may be desirable for ease of contact, and if the device issupported by that edge, there would be no need to round the first andsecond substrates 22, 26 along the contact edge 66. Any exposed edge onthe electro-optic assembly 10 may be generally rounded. The radius ofcurvature of the rounded edges 62 may be greater than approximately 2.5mm.

The electro-optic assembly 10 may include a transflective coating 70, anoptical coating 80, and a scratch-resistant coating 90. In the depictedexample, the transflective coating 70 is positioned proximate the firstsurface 22A, but may additionally or alternatively be positioned on thesecond surface 22B without departing from the teachings provided herein.In the depicted example, the optical coating 80 is on the first, thirdand fourth surfaces 22A, 26A, 26B, but it will be understood that theoptical coating 80 may additionally or alternatively be positioned onthe second surface 22B without departing from the teachings providedherein. The optical coatings 80 on the second and third surfaces 22B,26A, in certain examples, function as electrodes (e.g., anantireflective electrode) to enable darkening of electro-optic medium38. It will be understood, that when the transflective coating 70 islocated on the second surface 22B, in certain examples, it may alsoserve a dual purpose and also act as an electrode. In the depictedexample, the scratch-resistant coating 90 is positioned proximate thefirst and fourth surfaces 22A, 26B.

As described in greater detail below, the optical coating 80 may be usedto both lower the reflectance of a surface (e.g., the fourth surface26B) of a substrate (e.g., the second substrate 26) as viewed throughthe substrate, and/or to decrease the transmission of light through thesubstrate. The antireflective and transmission properties of the opticalcoating 80 may change as the thickness of the coating 80 is altered. Forexample, as the thickness of the optical coating 80 is increased, thereflection from the surface may be decreased to a minimum and then beginto rise again. In order to attain this characteristic, the opticalcoating 80 includes a thin coating (e.g., such as a metal) withappropriate optical constants which are explained in greater detailbelow. At the point at which the optical coating 80 has a thickness suchthat the reflection from the surface is substantially equal (e.g., ±lessthan about 10%, 5%, 3%, 2%, 1% absolute) to that of an uncoatedsubstrate, the transmission through the substrate and the opticalcoating 80 may be decreased (e.g., by greater than about 5%, 10%, 15%,20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60% absolute) relative to anuncoated portion of the substrate. In other words, the optical coating80 does not substantially affect the reflectivity of the fourth surface26B. Such a difference between the coated and uncoated surfaces may beknown as a reflectance deviation. Such an optical coating 80 with anequivalent reflectivity as the fourth surface 26B of the secondsubstrate 26, but a lower transmission, may be known as an equivalencelayer. A tradeoff may be obtained between reflectivity and transmissionby altering the refractive index and/or thickness of the optical coating80. The specific combinations of reflectance and transmittance will varydepending on the different examples (described below). The square of thetransmittance is a characteristic referred herein as the attenuationfactor (described below). In certain examples the attenuation factorshould be less than about 65%. It will be understood that althoughdescribed in connection with the electro-optic assembly 10, the opticalcoating 80 may equally be applied to any desired transparency (e.g.,individual substrates such as pieces of glass or plastic, windows, thewindscreen 54, packaging, etc.). By tailoring the optical coating 80 tohave a reflectivity substantially similar to that of the uncoatedsubstrate, but have a lower transmission, the optical coating 80 may notbe readily apparent to a viewer. Such a use may be advantageous inconcealing objects proximate and rearward to the optical coating 80 orfor generally decreasing the intensity of light passing through thesubstrate. Further, use of the optical coating 80 may be advantageous inthat the ability to use a single material for both antireflective anddecreased transmission applications may simplify manufacturing. Theoptical coating 80 may include metal layers, dielectric layers, andstacks including both (e.g., in an alternating manner).

Antireflective Optical Coatings

Unlike other traditional antireflective applications, it is important tonote that the problem being solved is not the reflectance as viewed fromthe fourth surface 26B, but rather, from the reverse direction (e.g.,from the third surface 26A) as shown in FIG. 4 simplified with a singlesubstrate (i.e., the second substrate 26). In other words, thereflection through the substrate should be minimized. For example, alight beam 39 incident on surface 26B may reflect as reflected beam 41.Beam 41 may have low reflectance, relative to an uncoated portion of thesecond substrate 26, due to the optical coating 80 on the fourth surface26B. A light beam 39′ directed toward surface 26B from the oppositedirection will have reflected beam 41′. The optical coating 80 is notrequired to provide antireflection properties from this directionopposite the viewer. Thus, the reflectance as viewed from the fourthsurface 26B actually does not have any reflectance constraints. Thisunique set of optical constraints can be solved with a new type ofoptical coating design based on thin layers including materials such asmetals. It has been discovered that the reflectance of a thin metalcoating will vary by the direction viewed. For example, when a Crcoating is applied to a glass substrate the reflectance from the coatingside will steadily increase. Conversely, the reflectance when viewedthrough the glass will have an alternate behavior. As the metal coatinglayer increases thickness, the reflectance drops initially, goes througha minimum before it steadily increases in reflectance as explainedabove. This effect occurs for very thin coating layers. An example ofthe reflectance in dependence of wavelength in the visible range isillustrated in FIG. 5 for a glass/air interface in the uncoated state, aglass/air interface with a four layer HL (alternating high and lowrefractive index) example of the optical coating 80, and a glass/airinterface with a thin chromium example of the optical coating 80. Fromthis, it is possible to observe that the thin metal layer reduces adramatic amount of reflectance from the glass as viewed from theobserver perspective.

It has been discovered that certain metals, materials or alloys willproduce an antireflection effect where the reflectance, as viewedthrough the substrate, is quite low. The antireflection effect may beobtained by a variety of metals or materials with unique opticalproperties. One such optical property is the refractive index. Therefractive indices of materials have a real component, n, and animaginary component, k. The antireflection effect for a series of metalsand alloys was analyzed using thin film modeling techniques to determinethe appropriate relationship between n and k which will provide adequateantireflection properties. The analysis looked at n and the ratio of nk.Table 1 provides a list of potential materials with refractive indicesfrom which the optical coating 80 may be formed.

TABLE 1 Materials studied, resultant optimal antireflection property,and the n, k and nk ratio for the material at 550 nm wavelength.Combined Transmittance 1st and 2nd Reflectance (2nd Reflectance Surface(2nd surface (from n′@ k′@ Reflectance surface interface rearwardThickness 550 550 nk Material Y optimized only) only) direction) (nm) nmnm ratio AlSi6040 4.58 0.46 68.05 10.84 1.39 3.13 4.49 0.70 AlSi80206.25 2.27 80.21 7.90 1.25 1.56 4.51 0.35 AlSi8515 6.70 2.76 83.92 6.971.03 1.29 4.67 0.28 AlSi9010 6.90 2.93 85.23 6.63 0.88 1.24 4.94 0.25AlTi5050 4.32 0.18 66.36 11.23 2.83 2.54 2.96 0.86 AlTi7030 4.31 0.1666.26 11.25 2.18 2.88 3.39 0.85 AlTiRu90-8-2 5.62 1.59 75.39 9.09 1.082.28 5.12 0.45 Cadmium 6.90 2.95 85.38 6.59 1.25 1.04 4.06 0.26 Cu 6.822.66 — — — 0.95 2.58 0.37 CuSn5050 5.63 1.47 — — — 1.87 4.13 0.45 CuZn7.53 3.37 — — — 0.59 2.85 0.21 Ge 5.09 0.93 — — — 4.62 2.09 2.21 Ir 5.281.12 — — — 2.23 4.31 0.52 Mo 4.19 0.03 65.53 11.42 1.71 3.78 3.52 1.07MoRe 4.16 0.00 65.35 11.46 0.93 4.92 4.88 1.01 MoRe-8 4.19 0.03 — — —3.95 4.10 0.96 MoSi2 5.03 0.87 — — — 4.67 2.33 2.00 MoTa-1 4.19 0.0365.47 11.44 1.77 3.49 3.61 0.97 MoTa-10 4.20 0.04 — — — 3.84 4.08 0.94MoTa-4 4.27 0.11 — — — 2.96 3.37 0.88 MoW-1 4.19 0.03 — — — 3.64 3.840.95 Ni2Si 4.40 0.24 — — — 1.95 2.23 0.87 Niobium 4.20 0.04 65.56 11.412.66 2.92 2.87 1.02 Palladium 5.84 1.68 — — — 1.64 3.85 0.43 Platinum5.08 0.92 — — — 2.13 3.71 0.57 Rhenium 4.34 0.18 — — — 4.25 3.06 1.39Rhodium 5.59 1.43 — — — 2.08 4.54 0.46 Ru 4.83 0.67 — — — 3.28 5.46 0.60Stainless Steel 4.71 0.55 — — — 2.44 3.61 0.68 Ta 4.17 0.01 65.38 11.451.81 3.54 3.48 1.02 W 4.17 0.07 65.75 11.36 2.28 3.65 3.71 0.98 V 4.220.06 — — — 3.68 3.01 1.22 Zn CRC 6.69 2.53 — — — 1.01 4.31 0.23 Zr CRC4.36 0.22 66.97 10.88 11.90  1.82 0.95 1.92

In each case, the thickness was optimized to minimize the reflectance.These values were then statistically analyzed to determine which n andnk ratio will result in low reflectance. The materials were positionedon the rearward surface of a glass substrate which has first and secondsurfaces. The reported reflectance values in Table 1 include the sum ofthe reflectance from both the first and second surfaces which were usedfor the statistical analysis. Additional data is present in the tablefor the second surface reflectance and transmittance which shows theoptical properties of the coated interface (the second surface) with thefirst surface properties absent (such as reflectance from the forwarddirection). Additionally, the rearward reflectance is presented in Table1 (which also does not include reflectance from the first surface 22A.The reflectance values reported are Y values in the CIE Yxy color systemwhich are weighted to the human eye's sensitivity to light intensity.The statistical analysis used the Y values for reflectance, but it willbe understood that the reflectance may alternatively be an average overa given wavelength range (or ranges), such as the visible wavelengthrange, or may be a relatively narrow wavelength band. A given examplemay provide antireflective properties at reflectance values (describedbelow) for one or more of these reflectance characterization methodswithout deviating from the teachings provided herein.

FIG. 6A depicts a contour plot with regions of different reflectancebased on n vs. nk ratio. The reflectance from the surface coated withthe optical coating 80 may be less than about 1.5%, 0.75%, 0.5%, 0.25%or less than about 0.1%. From this graphical representation of thestatistical analysis, to attain a desirable reflectance, the n value maybe greater than about 1.5, 2.0, 3.0 or greater than about 3.5. To obtaina desirable reflectance, the nk ratio may be greater than about 0.4,0.5, 0.6, 0.7, 0.8, 0.9 or 1.0. From FIG. 6A, in a specific example, then value may be greater than about 2.5 and the nk ratio between 0.6 and2.0 such that reflectance values less than about 0.5% are attainable. Inanother example, the n value may be greater than about 2.75 and the nkratio may be between about 0.8 and 1.75 which may result in reflectancevalues less than about 0.25%. Alternatively, the relationship between nand nk ratio may be expressed as simplified expressions. FIG. 6B showsseveral linear approximations for the regions in the contour plot whichcan be used to approximate the n and nk relationships needed to attain agiven reflectance level. If a reflectance of less than about 1.0% isdesired then the n and nk ratio should reside within the region definedby the equations; nk ratio+0.253*n<3.29 and nk ratio+1.118*n>1.02. If areflectance of less than about 0.5% is desired then the n and nk ratioshould reside within the region defined by the equations: nkratio+0.193*n<2.71 and nk ratio+1.121*n>1.18. If a reflectance of lessthan about 0.25% is desired then the n and nk ratio should reside withinthe region defined by the equations; nk ratio+0.216*n<2.60 and nkratio+1.126*n>1.29. The full statistical equation which will provide themost accurate relationship between reflectance, n and nk ratio is:Reflectance (second surface 22B only)=4.945−1.170*n−4.701*nkratio+0.1230*n²+1.345*nk ratio²+0.2785*n*nk ratio. This formula may beemployed for refinement of needed n and nk ratios to attain a givenreflectance after one finds the general requirements using the generalguidelines described above. It will be understood that the term “metal”is used herein to describe a material which meets the refractive indexrelationships defined above, but that the teachings are not limitedexplicitly to metals. Other materials meeting the refractive indexrequirements may be used and are within the scope of teachings providedherein. One skilled in the art will be aware that the refractive indicesof materials is not constant and that they vary over the visiblespectrum. Therefore, it can be expected that some optimization may beneeded in tuning the refractive index and thickness to get targetedreflectance values across the desired spectra. Suitable example metalsor alloys for the optical coating 80 which will have reflectance in aglass/air system include: AlSi alloys, AlTi alloys, AlTi5050, AlTi7030,chrome, Mo, MoRe alloys, MoSi2, Nb, Co, Ir, platinum, rhenium,ruthenium, stainless steel, Ta, W, V, and Zr or mixtures and alloys ofthese materials. Exemplary reflectance spectra for several materials areshown in FIG. 7. The reflectance spectra demonstrates that reflectancereduction occurs over a broad wavelength range.

The reflectance versus thickness for several metals when viewed througha substrate is illustrated in FIG. 8. The reflectance is for theeye-weighted (Y), normal incidence reflectance which includes thereflectance from the first, uncoated surface. From FIG. 8, it ispossible to observe that the metals show a characteristic minimum in thereflectance at a thickness between about 0.5 and about 4.0 nm and areduced reflectance up to 5 nm for some metals and alloys. Accordingly,when the optical coating 80 is being used as an antireflection coating,a thickness of the coating 80 may be between about 0.1 and 5 nm, orbetween about 0.2 and 4 nm, or between about 0.5 and 3.5 nm.

In an alternative example to minimize the reflectance of a surface(e.g., any surface of the first and second substrates 22, 26), a textureor roughness may be introduced to the optical interface between the twomedia. The feature dimensions of the texture may be in the order ofmagnitude of the light wavelength, such as in an anti-glare or moth-eyeanti-reflection surface. Combining an anti-glare texturized surface witha metal-based antireflection coating (e.g., the optical coating 80) maybe advantageous in reducing the reflectance of the interface to levelslower than that exhibited by the texturized surface or a metal-basedantireflection coating alone. The features of the textured or roughenedsurface may have a dimension of between about 0.1 μm and about 20 μm. Ahaze of the textured or roughened surface may be less than about 50%,less than about 30% or less than about 10%.

When coatings are used in applications where the coating is reflectingor transmitting a color image, it is important that the color renderingof the image is correct. Under some circumstances, it is desired thatthe colors are not modified by either being reflected, transmitted orboth through the coated substrate. The preservation of color can bequantified by either the color rendering index (CRI) or by the reflectedcolor C*. The CRI of the coated substrate may be greater than about 80,85, 90, 95 or greater than about 99. Alternatively, in units ofC*=√(a*²+b*²), where a* and b* are color parameters of the CIELAB colorsystem, the color of the optical coating 80 should have a value lessthan about 10, less than about 5 or most desirably less than about 2.5.Either of these metrics will describe a surface where the reflected ortransmitted colors will be true or approximately match those of theoutput device in either a reflected or transmitted configuration orboth. In other examples, the coating can be tuned to match the output ofthe display to enhance or compensate to achieve the desired colors.Table 2 shows selected optical coating 80 constituents from Table 1which includes reflected and transmitted color values and respective C*values. The data in Table 2 includes the contribution to the reflectanceand transmittance from the uncoated first surface. The optical coating80 taught herein may have good C* values and would therefore functionwell in preserving both reflected and transmitted colors.

TABLE 2 Integrated eye-weighted reflectance minima and correspondingtransmittance of single layer metallic antireflection coatings on glass.Metal layer thickness Material (nm) Yr a*r b*r C*r Yt a*t b*t C*tAbsorption Raw Glass 0 8.43 −0.23 −0.95 0.98 90.71 −0.33 0.27 0.43 0.86Chromium 1.48 4.74 −0.23 −3.66 3.67 63.57 0.45 −3.26 3.29 31.69 Cobalt2.02 5.59 −0.08 −1.37 1.37 68.88 −0.27 −1.36 1.39 25.54 Iridium 1.625.63 −0.59 −1.68 1.78 69.14 −0.52 −1.8 1.87 25.23 Mo 1.78 4.47 −0.04−1.11 1.11 61.98 −0.04 −1.11 1.11 33.55 MoRe-5 1.25 4.46 −0.04 −0.820.82 61.89 −0.16 1.75 1.76 33.65 MoTa-5 2.18 4.58 −0.04 −1 1.00 62.61−0.03 0.81 0.81 32.81 MoW-5 1.6 4.44 −0.03 −0.89 0.89 61.8 −0.1 1.981.98 33.76 Niobium 2.77 4.48 0.18 −1.17 1.18 62.03 1.62 2.24 2.76 33.49Platinum 2.13 5.42 −0.1 −1.04 1.04 67.75 −0.62 −1.4 1.53 26.83 Rhenium1.68 4.63 −0.09 −0.76 0.77 62.94 0.76 4.22 4.29 32.43 Tantalum 1.89 4.45−0.06 −0.98 0.98 61.83 −0.17 −0.14 0.22 33.73 Titanium 3.93 4.99 −0.22−1.13 1.15 65.07 −1.31 −0.85 1.56 29.94 Tungsten 1.71 4.45 −0.04 −1.011.01 61.87 −0.33 0.8 0.87 33.67 Vanadium 2.08 4.5 0.38 −1.24 1.30 61.162.01 3.11 3.70 33.34

As shown in Tables 1, 2 and FIG. 7, some of the examples of materialsfor the optical coating 80 may have reflectance values greater than zerofor their optimal antireflection situation. It is not uncommon forantireflective coatings to have some positive value of reflection as itcan be challenging to antireflect over a broad wavelength range. In someexamples, the optical coating 80 can be further improved by the additionof a thin dielectric, insulator or transparent conducting oxide layerpositioned between the substrate (e.g., the second substrate 26) and thematerial of the optical coating 80. Table 3 below shows the benefitsattainable for metal examples (e.g., chromium) of the optical coating 80using thin film models. As can be seen, the reflectance is reducedsubstantially with the addition of the dielectric layer. The desiredthickness and refractive index of this dielectric layer will vary withthe material of the optical coating 80 and the requirements of theapplication. The refractive index of the dielectric layer may be lessthan about 2.4, less than about 2.0. The thickness may be less than 50nm, or less than 35 nm.

TABLE 3 Cr Dielectric Dielectric Thickness Reflectance TransmittanceSample RI Thickness (nm) Y a* b* Y a* b* 1 — — 1.56 4.66 −0.33 −3.7165.50 0.61 −3.45 2 1.6 32.73 1.63 4.59 −0.23 −3.81 64.69 0.62 −3.55 31.7 31.51 1.87 4.40 0.16 −3.83 61.92 0.69 −3.94 4 1.8 22.42 2.04 4.280.51 −3.55 60.12 0.72 −4.22 5 1.9 22.90 2.13 4.23 0.73 −3.07 59.25 0.73−4.40

Unexpectedly, the use of the optical coating 80 can be used inconjunction with a relatively thick transparent conductive oxide (TCO)layer such as indium tin oxide (ITO) to create a low reflectancetransparent electrode suitable for use in electro-optic devices such asa HUD combiner for the heads-up display system 14. Table 4 below showsthe reflectance of an ITO layer which is 370 nm thick. The reflectanceoff the coated surface is 2.9%. In the examples in Table 4, the ITO andother coatings are on the rearward surface of a glass substrate and anelectrochromic material with a refractive index of about 1.44 is used asan exit media. The reflectance from the first surface is omitted. Theaddition of a thin tantalum layer as an example of the optical coating80 onto the ITO layer drops the reflectance to about ⅓ of its initiallevel. It will be understood that although results utilizing tantalumare provided, other compounds from Table 1 and elsewhere herein may beused with comparable results. The thickness of the TCO or ITO may beabout 10 nm, about 100 nm, 150 nm, 250 nm or 350 nm and all valuestherebetween. The third example in Table 4 includes an additional colorsuppression (CS) layer positioned between the substrate and the ITOlayer. In this case, the reflectance drops to less than about 0.1%. TheCS layer may have a refractive index of between about 1.6 to about 1.75,or between about 1.63 and 1.71. The example provided in Table 4 had anindex of 1.67 for the CS layer. The thickness of the CS is approximatelya quarter-wave optical thickness for a design wavelength between about450 and 600 nm. Alternatively, the CS layer may be a bi-layer includinga high and low index material, known as a Herpin equivalent index layer,whose net index and optical thickness matches the requirements listedabove. The fourth example in Table 4 includes a Herpin equivalent CSlayer composed of a bilayer of Nb₂O₅ and SiO₂ with a thin Molybdenum toplayer. The reflectance drops to under 0.1% and the sheet resistance isunder 4 ohms/sq. The fifth example shares the same material stack as thefourth example, with the main difference of having a much thinner ITOlayer, demonstrating the control in the sheet resistance and also areflectance under 0.1%. The sixth example demonstrates that one canobtain higher transmission by using a different metal thin layer on topof the ITO, while keeping the reflectance under 1%. Alternate CS layersinclude, but are not limited to, dual quarter wave stacks wherein theoptical thickness of each layer is approximately a quarterwave opticalthickness and the refractive indices are intermediate the refractiveindex of the glass and TCO. In another embodiment, the CS layer may be agraded index layer wherein the refractive index approximately matchesthe refractive index of the substrate at the substrate/CS interface andapproximately matches the index of the TCO at the CS/TCO interface andthe refractive index gradually changes throughout the thickness of theCS. In yet another example, the CS layer may be a digitized graded indexlayer wherein the graded index is achieved by thin alternating layers ofhigh and low refractive index materials. The reflectance spectra for theexamples in Table 4 are shown in FIG. 9. It will be understood thatother intermediate layers may be present or used and that someoptimization may be desired to balance different design goals. It willbe understood that R (%) and T (%) in Table 4 represent percentreflectance and percent transmittance, respectively.

TABLE 4 R (%) R (%) 2^(nd) surface 2^(nd) surface Sheet Material viewedT viewed Thicknesses resistance Example stack from front (%) from back(nm) (Ohm/sq) 1 ITO 2.90 92.77 3.00 370 4.2 2 ITO/Ta 0.87 74.86 6.47370/1.6 4.2 3 CS/ITO/Ta 0.09 75.44 4.74 82/370/1.6 4.2 4Nb₂O₅/SiO₂/ITO/Mo 0.02 73.45 4.44 9/32/432/1.3 3.6 5 Nb₂O₅/SiO₂/ITO/Mo0.01 85.24 2.97 7/43/20/0.8 77 6 Nb₂O₅/SiO₂/ITO/Cu 0.94 87.11 2.359/29/432/1.6 3.6

Alternatively, the reflectance of the optical coating 80 can be reducedby the addition of a dielectric, an insulator or transparent conductingoxide layer positioned above a metal layer of the optical coating 80.Table 5 shows the change in reflectance with the addition of an ITOlayer above a relatively poorly antireflecting metal. Additionally, astructure with metal and ITO layers both above and below it are shown.The change in the reflectance spectra for the metal alone and withdielectric layer or layers are shown in FIG. 10 for select examples. Theintensity of the reflectance is decreased across the plotted spectra.The thickness of the dielectric layer in this example is less than about25 nm and or may be less than about 18 nm.

TABLE 5 Reflectance Reflectance Thick- (2nd Surface Transmit- fromReverse nesses Material Interface Only) tance Side (nm) Glass/ 0.0066.67 10.58 1.53/6.7  AlSi6040/ ITO Glass/ 0.03 70.27 8.40 1.61/13.24AlTiRu 90- 8-2/ITO Glass/ 0.08 73.38 6.80 2.32/16.37 AlSi8020/ ITOGlass/Cr/ 0.05 64.55 11.20 1.93/7.37  ITO Glass/ITO/ 0.03 64.05 11.862.69/ Cr 5% air/ 1.26/1.58 ITO

The reflectance of the metal or dielectric metal stacks of the opticalcoating 80 may be further reduced by the modification of the refractiveindices of the metal layers. This can be accomplished by the addition ofsmall dopants or additives to the metals such as nitrogen, oxygen, bothor other elements. For example, a chromium layer was sputtered with 5%oxygen and 5% nitrogen and the reflectance of the coated surface was0.10% for both cases. Other levels of gasses may be used in thesputtering atmosphere to change the optical properties of the metals.The percentages of the dopant gas sources can be varied experimentallyto optimize the reflectance as needed. The refractive index relationshipdescribed above can be used to guide the optimization of the materialsfor the desired antireflection properties. Additional examples are shownin Table 6. The reflectance of an un-doped chromium antireflection layerexample of the optical coating 80 is approximately 0.54% (eye-weighted).When dopants are added the reflectance drops to less than about 0.15%.The change in reflectance follows the rules for n and the nk ratio, asexplained above. As shown in Table 6, the doped chrome n and nk valuesapproach a more preferred state as shown in the contour plot of FIG. 6.The reflectance spectra for several examples in Table 6 are shown inFIG. 11. The addition of a small amount of dopants may reduce theoverall reflectance across the plotted spectrum.

TABLE 6 Combined Transmittance 1st and 2nd Reflectance (2nd ReflectanceSurface (2nd surface (from n′@ k′@ Reflectance surface interfacerearward Thickness 550 550 nk Material Y optimized only) only)direction) (nm) nm nm ratio Cr — 0.54 68.58 10.72 1.54 2.96 4.28 0.69 Cr5% Air — 0.11 65.96 11.32 1.15 4.13 4.67 0.88 Cr 15% Air 4.28 0.14 66.1011.29 1.32 3.75 4.40 0.85 Cr 5% O2 0.10 64.92 11.67 1.04 4.52 4.95 0.91Cr 10% O2 4.30 0.15 66.18 11.27 0.93 4.48 5.25 0.85 Cr 5% N2 — 0.1065.91 11.33 1.17 4.12 4.60 0.90

The modeled structures of FIG. 11 and Table 6 are for the case where themetal is positioned on a substrate and has air as an exit media. Therole of the exit media was examined and Table 7 shows that lowreflectance can be obtained when the exit media refractive index isincreased. For example, the exit media could be a liquid fluid or solidmaterial rather than air or gas.

TABLE 7 Reflectance Transmittance Reflectance Thick- (2nd surface (2ndsurface (from rearward ness Material only) interface only) direction)(nm) W 1.1 exit 0.04 72.20 7.36 1.84 W 1.2 exit 0.02 78.68 4.22 1.39 W1.3 exit 0.01 85.12 1.95 0.95 W 1.4 exit 0.00 91.48 0.55 0.51

The optical coating 80, on the fourth surface 26B, may acquire a buildupof environmental contaminants or dirt which is common in an automotiveinterior. The optical coating 80 may therefore be subjected to regularcleaning to have the best images possible. If the optical coating 80 isnot durable then it may be scratched or otherwise damaged by thecleaning solvents or methods. Accordingly, it may be advantageous forthe optical coating 80 to be durable. If additional durability is neededfor the optical coating 80, the scratch-resistant coating 90 may beadded to the optical coating 80. According to one example, thescratch-resistant coating 90 may be a diamond-like carbon (DLC) layer.Since the DLC layer typically has a relatively high refractive index,the layers of the optical coatings 80 may need to be optimized oradjusted to attain the desired balance between reflectance anddurability. Table 8 shows reflectance for several thin metal examples ofthe optical coating 80 with a DLC overcoat example of thescratch-resistant coating 90. The reflectance spectra for select metalswith DLC overcoats are shown in FIG. 12.

TABLE 8 Reflectance Thick- (back Transmit- Back nesses Material surface)tance Reflectance (nm) W + DLC 0.12 66.36 11.17 2.11/0.5 Cr 5% air +0.03 65.01 11.57  1.1/1.0 DLC AlTiRu 90- 0.00 66.59 10.62 0.97/5.0 8-2 +DLC

Equivalence Layer Optical Coatings

As explained above, an optical coating 80 with an equivalentreflectivity as the fourth surface 26B of the second substrate 26, but alower transmission, may be known as an equivalence layer. The opticalcoating 80 will continue to have a reduced transmittance as itsthickness increases. As shown in FIG. 8, the reflectance will increasepast a minimum and, at some thickness, will be essentially equivalent tothe reflectance of the fourth surface 26B of the second substrate 26.The thickness and subsequent transmittance at this equivalence pointwill be dependent on the material or metal (e.g., the metal layer of theoptical coating 80) and/or additional layers (e.g., dielectric,insulator or transparent conducting oxide layers and scratch-resistantcoatings 90). As the thickness of the optical coating 80 is furtherincreased, the reflectance will be higher than that of the secondsubstrate 26. This equivalence point introduces unique attributes to theoptical coating 80 which may be utilized. At the equivalence point, thereflectance of the surface (e.g., the fourth surface 26B) of thesubstrate (e.g., the second substrate 26) may be essentially equivalentto the uncoated substrate while having a lower transmittance. Asexplained above, the absolute reflectance of the coated substrate,viewed from the side opposite the coating, should be within thereflectance of the uncoated substrate by less than about 10%, 5%, 2.5%,1% (absolute) and the transmittance of the coated substrate may bereduced by more than about 20%, 35%, 50%, 60% relative to the uncoatedsubstrate.

A similar analysis to the reflectance was performed wherein thethickness of the optical coating 80 was optimized so that thereflectance from the substrate side matches that of the uncoatedsubstrate (e.g., glass). Table 9 shows a series of metals, alloys andelements which were optimized to match the reflectance of the uncoatedglass substrate. In this example, the transmittance was calculated todetermine the amount of transmittance that would be possible fordifferent metal constituents of the optical coating 80. A lightattenuation factor is the square of the transmittance of the coatedglass since the light will pass through the coating twice (i.e., fromthe third surface 26A, through the optical coating 80, incident on anobject, back through the coating 80, and back through the third surface26A). Therefore, a 60% transmittance coated glass will have anattenuation factor of about 36% while a 50% transmittance coated glasswill have an attenuation factor of about 25%. The light attenuationfactor for the coating 80 may be less than about 50%, less than 35 orless than about 20%.

TABLE 9 Thickness Material n k nk ratio Transmittance Attenuation Factor(nm) AlSi6040 3.13 4.49 0.70 43.10 19% 3.45 AlSi8020 1.56 4.51 0.3562.35 39% 2.80 AlSi8515 1.29 4.67 0.28 69.00 48% 2.26 AlSi9010 1.24 4.940.25 71.40 51% 1.92 AlTi5050 2.54 2.96 0.86 40.74 17% 7.10 AlTi7030 2.883.39 0.85 40.59 16% 5.48 AlTiRu90-8-2 2.28 5.12 0.45 54.19 29% 2.52Cadmium 1.04 4.06 0.26 71.62 51% 2.74 Cr 10% O₂ 4.48 5.25 0.85 40.57 16%2.36 Cr 15% Air 3.75 4.40 0.85 40.42 16% 3.32 Cu 0.95 2.58 0.37 71.7251% 5.46 CuSn5050 1.87 4.13 0.45 54.40 30% 3.76 CuZn 0.59 2.85 0.2178.58 62% 3.82 Ge 4.62 2.09 2.21 49.05 24% 4.90 Ir 2.23 4.31 0.52 50.2925% 3.68 Mo 3.78 3.52 1.07 39.56 16% 4.37 MoRe 4.92 4.88 1.01 39.38 16%2.37 MoRe-8 3.95 4.10 0.96 39.57 16% 3.53 MoSi₂ 4.67 2.33 2.00 48.06 23%3.64 MoTa-1 3.49 3.61 0.97 39.60 16% 4.48 MoTa-10 3.84 4.08 0.94 39.6216% 3.61 MoTa-4 2.96 3.37 0.88 40.25 16% 5.48 MoW-1 3.64 3.84 0.95 39.6216% 4.02 Ni₂Si 1.95 2.23 0.87 41.40 17% 11.82 Ni 2.92 2.87 1.02 50.1025% 6.07 Palladium 1.64 3.85 0.43 56.97 32% 4.12 Platinum 2.13 3.71 0.5748.15 23% 4.87 Rhenium 4.25 3.06 1.39 40.84 17% 4.12 Rhodium 2.08 4.540.46 53.83 29% 3.16 Ru 3.28 5.46 0.60 45.58 21% 2.40 Stainless Steel2.44 3.61 0.68 44.22 20% 5.20 Ta 3.54 3.48 1.02 39.50 16% 4.60 W 3.653.71 0.98 39.97 16% 5.80 V 3.68 3.01 1.22 39.69 16% 5.20 Zn CRC 1.014.31 0.23 69.24 48% 2.69 Zr CRC 1.82 0.95 1.92 42.00 18% 29.87

FIG. 13 shows the transmittance at perfect equivalence for the coatedand uncoated substrates with respect to the n vs nk ratio. As can beseen, the refractive index of the equivalence coating may have a realpart of the refractive index, n, which is greater than about 1.3, 1.5,1.75, 2.0, 2.5, 3.0 or 3.5 with a nk ratio of greater than about 0.15,0.3, 0.6, 1.0, 1.5 or 2.0. In a specific example, n may be greater thanabout 1.6 and the nk ratio may be greater than about 0.4 for atransmittance less than about 60% and an attenuation factor of less than36%. In another example, n may be greater than 1.8 and the nk ratio maybe greater than about 0.6 for a transmittance less than about 53% and anattenuation factor of less than 28%. In yet another example, n isgreater than about 2.5 and the nk ratio is greater than about 0.75 for atransmittance less than about 45% and an attenuation factor of less than20%.

Referring now to FIG. 14, depicted is an example of the electro-opticassembly 10 utilizing an equivalence layer example of the opticalcoating 80. The first substrate 22 has a perimeter spectral ring 108designed with low transmittance to hide the seal 30. The secondsubstrate 26 is bonded to the first substrate 22 with the seal 30 tocreate a chamber which is filled with the electro-optic medium 38. Thesecond substrate 26 has disposed on it the transflective coating 70. Anelectrical connection to the electro-optic medium 38 is attained byJ-clips 112 which wrap around the edge of the second substrate 26 toallow connection between the third and fourth surfaces 26A, 26B. Theoptical coating 80 is positioned on the fourth surface 26B between oneof the J-clips 112 and the second substrate 26. A first light beam 116is depicted as incident on the electro-optic assembly 10 and passingthrough the first and second substrates 22, 26. The first light beam 116reflects off the J-clip 112 and again passes through the first andsecond substrates 22, 26 to leave a first reflected beam 116′. Becausethe transflective coating 70 is transflective, there can be substantialreflectance from the J-clip 112 which will then be visible as the firstreflected beam 116′. A second light beam 120 is also incident on theelectro-optic assembly 10, but passes through the equivalence layerexample of the optical coating 80. As the reflectance of equivalencelayer example of the optical coating 80 is designed to match thereflectance of the uncoated portion of the fourth surface 26B, therewill be no increase in light intensity of the second reflected beam120′. The intensity of the second light beam 120 will be reduced as itpasses through the optical coating 80. It will then reflect off J-clip112 and pass again through the optical coating 80. Finally, the secondreflected light beam 120′ will emerge from the electro-optic assemblywith substantially reduced intensity compared to the first reflectedbeam 116′ which does not pass through the optical coating 80. In thedepicted example, the optical coating 80 is a chrome layer with anapproximate thickness of about 3.8 nm and has a reflectanceapproximately equal to the reflectance of the further surface 26B. Atthis thickness, the transmittance through the optical coating 80 isapproximately 44%. The net attenuation factor of the second light beam120 and the second reflected light beam 120′ passing through this layertwice is the product of 44% times 44% which is approximately 20%. Theintensity of the reflectance off the J-clip 112 is therefore reduced bya factor of 5 from its original value. The level of the attenuationfactor needed will depend on a given application, in particular on thereflectivity of the rearward components and the lighting conditionsunder which the electro-optic assembly 10 is being viewed.

Examples of materials, metallic elements and alloys suitable for theequivalence coating would be a single or multi-layer of a metallicmaterial such as those elements and alloys shown in the table above oralloys containing these elements which have a transmittance of less thanabout 60%. The thickness of the metal layers may be between about 1 andabout 20 nm, between about 1 and about 9 nm, or between about 3 andabout 8 nm. Similarly to the use of thin materials or metallic layersfor antireflection properties, the equivalence coating may include themetals and/or alloys listed above along with additional insulator,dielectric, TCO, DLC or other metals configured to meet the reflectanceand light attenuation objectives.

Use of the present disclosure may offer a variety of advantages. First,the optical coating 80 of the disclosure may be used as both anantireflective coating as well as a coating to decrease transmission.Use of the same material for two purposes may decrease manufacturingcosts associated with the electro-optic assembly 10. Second, as theoptical coating 80 has substantially the same reflection as an uncoatedsubstrate, the optical coating 80 may go unnoticed, or have a “stealthy”appearance, when viewed by a person. For example, a discernible changein the reflectivity of the fourth surface 26B of the second substrate 26may not be noticed by a viewer, only the decreased transmission. Inother words, the optical coating 80 may not substantially affect (e.g.,±about 10%) the reflectivity of the fourth surface 26B. Third, theoptical coating 80, as described herein, may be applied to a widevariety of transparencies as explained above.

It will be understood by one having ordinary skill in the art thatconstruction of the described disclosure and other components is notlimited to any specific material. Other exemplary embodiments of thedisclosure disclosed herein may be formed from a wide variety ofmaterials, unless described otherwise herein.

For purposes of this disclosure, the term “coupled” (in all of itsforms: couple, coupling, coupled, etc.) generally means the joining oftwo components (electrical or mechanical) directly or indirectly to oneanother. Such joining may be stationary in nature or movable in nature.Such joining may be achieved with the two components (electrical ormechanical) and any additional intermediate members being integrallyformed as a single unitary body with one another or with the twocomponents. Such joining may be permanent in nature or may be removableor releasable in nature unless otherwise stated.

It is also important to note that the construction and arrangement ofthe elements of the disclosure, as shown in the exemplary embodiments,is illustrative only. Although only a few embodiments of the presentinnovations have been described in detail in this disclosure, thoseskilled in the art who review this disclosure will readily appreciatethat many modifications are possible (e.g., variations in sizes,dimensions, structures, shapes and proportions of the various elements,values of parameters, mounting arrangements, use of materials, colors,orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter recited. For example,elements shown as integrally formed may be constructed of multipleparts, or elements shown as multiple parts may be integrally formed, theoperation of the interfaces may be reversed or otherwise varied, thelength or width of the structures and/or members or connector or otherelements of the system may be varied, the nature or number of adjustmentpositions provided between the elements may be varied. It should benoted that the elements and/or assemblies of the system may beconstructed from any of a wide variety of materials that providesufficient strength or durability, in any of a wide variety of colors,textures, and combinations. Accordingly, all such modifications areintended to be included within the scope of the present innovations.Other substitutions, modifications, changes, and omissions may be madein the design, operating conditions, and arrangement of the desired andother exemplary embodiments without departing from the spirit of thepresent innovations.

It will be understood that any described processes or steps withindescribed processes may be combined with other disclosed processes orsteps to form structures within the scope of the present disclosure. Theexemplary structures and processes disclosed herein are for illustrativepurposes and are not to be construed as limiting.

It is also to be understood that variations and modifications can bemade on the aforementioned structures and methods without departing fromthe concepts of the present disclosure, and further it is to beunderstood that such concepts are intended to be covered by thefollowing claims unless these claims by their language expressly stateotherwise.

What is claimed is:
 1. A transparency, comprising: a first substrate comprising a first surface and a second surface; a second substrate comprising a third surface and a fourth surface; an optical coating positioned on the fourth surface having a refractive index real component n of greater than about 1.6 and an nk ratio greater than about 0.4, both as measured at 550 nm, and wherein the optical coating is configured to attenuate the transmission of the second substrate and not substantially affect the reflectivity of the fourth surface as viewed from the first surface of the transparency such that the attenuation factor is less than about 50%.
 2. The transparency of claim 1, wherein the optical coating has a refractive index real component n of greater than about 1.8 and an nk ratio of greater than about 0.6, both as measured at 550 nm, and an attenuation factor of less than about 35%.
 3. The transparency of claim 1, wherein a thickness of the optical coating is between about 1.5 nm and 15 nm.
 4. The transparency of claim 1, further comprising: a diamond-like carbon coating positioned on the optical coating.
 5. The transparency of claim 1, wherein a reflectance deviation of the fourth surface with the optical coating is less than about 5% absolute.
 6. The transparency of claim 1, further comprising: an electro-optic medium positioned between the second surface of the first substrate and the third surface of the second substrate.
 7. The transparency of claim 1, wherein the optical coating comprises a material selected from aluminum silicon alloys, aluminum titanium alloys, aluminum titanium ruthenium alloys, copper tin alloys, chrome, molybdenum rhenium alloys, molybdenum silicon alloys, molybdenum tantalum alloys, stainless steel, molybdenum tungsten alloys, nickel silicon alloys, metals or metal alloys selected from niobium, germanium, iridium, molybdenum, palladium, platinum, rhenium, rhodium, ruthenium, tantalum, cobalt, tungsten, vanadium, and zirconium, and combinations thereof.
 8. The transparency of claim 1, wherein the optical coating comprises a thickness of from about 0.1 nm to about 5 nm and an eye-weighted reflectance of less than about 9%, as viewed through the second substrate.
 9. The transparency of claim 1, wherein the optical coating comprises a reflected color C* value in the CIELAB color system of less than about
 10. 10. The transparency of claim 1, further comprising: a dielectric insulating layer disposed between the optical coating and the second substrate.
 11. The transparency of claim 10, wherein the dielectric insulating layer comprises a refractive index real component n of less than about 2.4, as measured at 550 nm.
 12. The transparency of claim 10, wherein the dielectric insulating layer has a thickness of less than about 50 nm.
 13. The transparency of claim 1, further comprising: one of a dielectric material, insulator, and a transparent conductive oxide layer disposed on the optical coating, on a side of the optical coating opposite the second substrate.
 14. The transparency of claim 1, wherein the optical coating comprises a metal doped with at least one of oxygen and nitrogen.
 15. The transparency of claim 1, wherein the optical coating comprises at least one chrome layer having a thickness of from about 1 nm to about 20 nm.
 16. The transparency of claim 1, further comprising: a first electrode disposed on the second surface and a second electrode disposed on the third surface.
 17. The transparency of claim 1, further comprising: an anti-reflective electrode disposed on the second surface and comprising a color suppression layer disposed proximate the second surface and a transparent conductive oxide disposed between the color suppression layer and the third surface.
 18. The transparency of claim 1, wherein the fourth surface comprises a first portion including the optical coating and a second portion that is free of the optical coating.
 19. The transparency of claim 18, wherein an absolute reflectance of the first portion is within about 10% of an absolute reflectance of the second portion and a transmittance of the first portion is less than a transmittance of the second portion, as viewed through the second substrate.
 20. The transparency of claim 1, wherein the transparency is configured to reflect an image from a projector of a heads-up display system of a vehicle. 